In general, the invention relates to desalination or other purification of water using hydrates to extract fresh water from saline or polluted water. In particular, the invention relates to desalination or purification of saline or polluted water using, e.g., industrial waste gas as a source of carbon dioxide; capturing the carbon dioxide from the source gas by forming carbon dioxide hydrate; and producing liquid carbon dioxide directly as a desalination byproduct.
In general, desalination and purification of saline or polluted water using buoyant gas hydrates is known in the art. See, for example, U.S. Pat. No. 5,873,262 and accepted South African Patent Application No. 98/5681, the disclosures of which are incorporated by reference. According to this approach to water desalination or purification, a gas or mixture of gases which spontaneously forms buoyant gas hydrate when mixed with water at sufficiently high depth-induced pressures and/or sufficiently low temperatures is mixed with water to be treated at the relatively deep base of a treatment column. According to prior technology, the treatment column is located at sea. Because the hydrate is positively buoyant, it rises though the column into warmer water and lower pressures. As the hydrate rises, it becomes unstable and disassociates into pure water and the positively buoyant hydrate-forming gas or gas mixture. The purified water is then extracted and the gas is processed and reused for subsequent cycles of hydrate formation. (Where the wet gas may be used for some other purpose, such as power generation nearby, it may prove unnecessary to process the gas and instead to use the gas in a pass-through mode; in this way, only the small amount of gas not recovered is an operating cost.) Suitable gases include, among others, methane, ethane, propane, butane, and mixtures thereof.
The previously known methods of desalination or purification using buoyant gas hydrates rely on the naturally high pressures and naturally low temperatures that are found at open ocean depths below 450 to 500 meters when using pure methane, or somewhat shallower when using mixed gases to enlarge the hydrate stability xe2x80x9cenvelope,xe2x80x9d and the desalination installations are essentially immobile once constructed, being fixed to pipelines carrying fresh water to land. In certain marine locations such as the Mediterranean Sea, however, the water is not cold enough for the requisite pressure to be found at a shallow enough depth; this would necessitate using a much longer column, which is impractical. Moreover, many places where fresh water is at a premium are located adjacent to wide, shallow water continental shelves where a marine desalination apparatus would have to be located a great distance offshore. Furthermore, a fixed installation is somewhat less versatile than a mobile installation would be. Additionally, the known methodologies have all required the hydrate, per se, to be buoyant in order to collect the hydrate and the fresh water released therefrom in an efficient manner.
In addition to using hydrates for desalination, it is also known to use hydrates to capture carbon dioxide from gas mixtures such as power plant emissions formed by burning fossil fuels by selectively forming carbon dioxide hydrates and then disposing of the carbon dioxide hydrates in an environment where the hydrate remains stable, e.g., at the bottom of the ocean. See, for example, U.S. Pat. Nos. 5,660,603, 5,562,891, and 5,397,553. Although such deep-sea disposal of carbon dioxide in the form of hydrate might be economically feasible where the hydrate is formed at or near the sea (e.g., on an oil rig or aboard a ship or at a seaside factory), it is far less economically feasible when the hydrate is produced inland. This is because of the expense of transporting the carbon dioxide-containing hydrate to the disposal location, the substantial majority of the weight and volume of which hydrate consists of water. (Carbon dioxide hydrate, like methane hydrate and other type I hydrates contains about 85% water on a molecular basis. In other words, about 85% of the molecules in these hydrates are water and about 15% of the molecules are gas molecules. The exact proportions vary slightly and are related to the degree of occupation of the xe2x80x98guestxe2x80x99 lattice sites in which the gas molecules reside.) Thus, the cost of such deep-sea disposal of carbon dioxide-bearing hydrate is increased substantially due to the cost of transporting (unnecessarily, as demonstrated by the present invention) the additional weight and volume of the water in the hydrate. Additionally, the previously known teachings of disposing of carbon dioxide via carbon dioxide hydrate completely ignores and therefore fails to take advantage of the tremendous capacity to obtain desalinated or otherwise purified water by means of forming and then melting (i.e., causing to dissociate) the carbon dioxide hydrates.
The various inventions disclosed herein overcome one or more of the limitations associated with the prior art and greatly expand the use and benefits of the hydrate desalination fractionation method by providing for land-based or mobile installation-based desalination of seawater (or other purification of polluted water) that is supplied to the installation and using either positively or negatively buoyant hydrate. The methods of the invention can be employed where input water is too warm or where suitably deep ocean depths are not available within reasonable distances for ocean-based desalination to be performed using gas hydrate, and may be carried out using a gas or gas mixture or even a liquid which produces either positively or negatively buoyant hydrate. Additionally, the invention can be practiced using carbon dioxide obtained from, e.g., industrial exhaust gases, thereby simultaneously providing purified water and capturing the carbon dioxide in the most efficient form for disposal (or other use, if so desired).
The inventive methods entail cooling the seawater to sufficiently low temperatures for hydrate to form at the bottom of a desalination fractionation column at pressure-depths and temperatures appropriate for the particular hydrate-forming material being used. A preferred embodiment capitalizes on the property of the hydrate that the amount of heat given off during formation of the hydrate at depth is essentially equal to the amount of heat absorbed by the hydrate as it disassociates (melts) back into pure water and a hydrate-forming material. In particular, as liquid or gas forms hydrate, and as the hydrate crystals rise through the water column (either due to inherent buoyancy of the hydrate or xe2x80x9cassistedxe2x80x9d by gas trapped within a hydrate mesh shell) and continue to grow, heat released during formation of the hydrate will heat the surrounding seawater in the column. As the hydrate rises in the water column and pressure on it decreases, the hydrate dissociates endothermicallyxe2x80x94the hydrate formation is driven primarily by the increased pressure at depthxe2x80x94and absorbs heat from the surrounding water column. Ordinarily, the heat energy absorbed during dissociation of the hydrate would be essentially the same heat energy released during exothermic formation of the hydrate such that there would be essentially no net change in the amount of heat energy in the system.
According to the invention, however, heat energy that is liberated during formation of the hydrate is removed from the system by removing residual saline water from the water column, which residual saline water has been heated by the heat energy released during exothermic formation of the hydrate. Because formation of the hydrate is primarily pressure driven (as opposed to temperature driven), the hydrate becomes unstable under reduced pressures as it rises through the water column, and it dissociates endothermically. Because some heat energy released during exothermic crystallization has been removed from the system, the hydrate will absorb heat from other sources as it melts, thereby creating a cooling bias. The preferred embodiment of the invention capitalizes on this cooling bias by passing the source water through the dissociation region of the water column, in heat-exchanging relationship therewith, so as to cool the source or supply water to temperatures sufficiently low for hydrate to form at the base of the installation.
As noted above, the invention may be practiced using liquid, gas, or gas mixtures which produce either positively buoyant hydrate or negatively buoyant hydrate. In the case of positively buoyant hydrate, the hydrate crystals themselves are positively buoyant and will rise naturally upon formation, upwardly through a desalination fractionation column at the top of which the hydrate disassociates into fresh water and the gas or gas mixture. In the case of negatively buoyant hydrate, on the other hand, the hydrate crystals, per se, are denser than the surrounding seawater and therefore ordinarily would tend to sink. By controlling the injection of the gas (or gas mixture) or liquid which produces the hydrate such that hydrate formation is incomplete, bubbles of the gas or the less-dense-than-seawater liquid are trapped within a mesh shell of hydrate, and overall positive buoyancy of the shell will cause the hydrate to rise within the water column.
Preferably, the rising assisted-buoyancy hydrate (negatively buoyant hydrate intimately intermixed with positively buoyant gas or liquid bubbles) is diverted laterally over a xe2x80x9ccatch basinxe2x80x9d so that the hydrate does not fall back down to the formation portion of the desalination fractionation column once the mesh shell disintegrates during dissociation. Solid, negatively buoyant hydrate, which has settled to a catch sump at the base of the apparatus, is pumped to the top of the catch basin, where it dissociates into gas and fresh water. (If so desired, forming the negatively buoyant hydrate in a slightly different manner will cause all the hydrate to settle in the sump, from which it is pumped to the dissociation/heat exchange catch basin.)
In alternative embodiments of the invention, the input water may or may not be passed through the dissociating hydrate in heat-exchanging relationship therewith to be cooled. In either case, the input water is (further) cooled using other, artificial means of refrigeration, the degree to which such cooling is necessary being in part a function of the buoyant or non-buoyant nature of the hydrate. Some heat energy is removed from the system by removing warmed water which has circulated around the desalination fractionation column in a water jacket and which has been heated by heat released during hydrate formation.
In the various embodiments of the invention, the purified water will be extremely cool. Advantageously, this cooled water, which preferably will be used as potable water, can itself be used as a heat sink to provide cooling, e.g., refrigeration as a basis for air conditioning in hot climates.
An additional advantage of land-based desalination or water purification according to the invention is that the installation is not subject to disturbances caused by foul weather and bad sea conditions nearly to the same extent as a marine site might be. Additionally, access to an installation on land is far easier than access to a marine-based installation. Gas handling and storage facilities are more practicable on land, where there is more space and a more secure engineering environment available. Construction is easier on land, and security may be improved as compared to a marine-based installation.
Moreover, because considerable amounts of residual seawater may be extracted from the system (to remove heat energy from the system), the hydrate slurry will be concentrated. This means that there will be less saline water in the upper, dissociation regions of the dissociation fractionation column, and therefore there will be less residual seawater for the hydrate to mix with as it dissociates. Thus, less salt will be present to contaminate the fresh water produced by dissociation of the hydrate.
Furthermore, because the residual seawater preferably is recirculated through the desalination fractionation column one or more times, other components such as trace elements which are in the seawater (e.g., gold) may be concentrated so that recovery from the seawater becomes practical. Additionally, the concentrated seawater may itself be useful or desirable. For example, marine aquarists might purchase such concentrated seawater to use for mixing replacement water for their aquaria, and such concentrated seawater would facilitate recreating the specific microcosm from which it was extracted.
In further embodiments, the desalination installation is constructed so that the hydrate dissociation occurs while the hydrate is still at some depth, such that it is still under considerable pressure, and the hydrate-forming gas is captured at this depth and processed for re-use while still at such pressures. Considerable efficiencies of operation are obtained with such arrangements.
In still further embodiments of the invention, rather than relying on the weight of a long column of water to create pressures appropriate for hydrate formation, the inventive methods may be practiced using self-contained, sealed hydrate formation/separation and hydrate dissociation/heat exchange vessels, which vessels are mechanically pressurized using appropriate hydraulic pumping systems. Such mechanically pressurized installations can be made comparatively mobile and may be used to provide the benefits of the invention in highly diverse settings. Moreover, such mechanically pressurized systems can be used to control the dissociation of the hydrate so that, when using carbon dioxide as the hydrate-forming medium, for examplexe2x80x94particularly when the carbon dioxide is provided by means of and is to be captured and sequestered from exhaust gases (e.g., industrial source exhaust gases)xe2x80x94the carbon dioxide released upon hydrate dissociation can be in either the liquid state or the gaseous state.